1. Field of the Invention
This invention relates to a process for producing a crosslinked and oriented polyethylene film and more particularly, it is concerned with a process for producing a crosslinked and oriented polyethylene film which is superior in clarity as well as moisture proofness and has an improved stretchability.
2. Description of the Prior Art
Polyethylene has hitherto been applied to a number of uses, but polyethylene films obtained from high density polyethylene have hardly been used in application areas where display effects are required mainly because they are generally poor in clarity. As a process for improving the clarity of polyethylene films, for example, it has been proposed to subject a polyethylene film to an irradiation treatment for uniformly crosslinking and biaxially orienting, as disclosed in Japanese Patent Publication No. 18893/1962.
This process is effective in improving the clarity and strength of the film, but not completely effective in improving the moistureproofness of the film. In order to impart improved moistureproofness, it is necessary to increase the film thickness or to provide the film with a resin layer as a barrier. This adversely affects the clarity and packaging property of the film and also results in an increased production cost.
The inventors have proposed a polyethylene film which is crosslinked and oriented in such a manner that the degree of crosslinking inwardly decreased across the thickness of the film, and a process for producing the same, thus obtaining a crosslinked and oriented polyethylene film excellent in clarity and moistureproofness, as disclosed in Japanese Patent Laid-Open Publication Nos. 174321/1984 and 174322/1984.
However, the thus obtained film has parts which are not crosslinked or crosslinked to a lower extent and consequently, tends to be affected by the structural properties of the original resin. Therefore, the above proposed process still leaves room for improvement in the production of the film, in particular, stretching operations such as controlling the stretching temperature, stock sheet thickness, film forming speed, etc.